The present embodiments relate generally to a cement composition for sealing a subterranean zone penetrated by a wellbore.
In the drilling and completion of an oil or gas well, a cement composition is often introduced in the wellbore for cementing pipe string or casing. In this process, known as “primary cementing,” the cement composition is pumped into the annular space between the walls of the wellbore and the casing. The cement composition sets in the annular space, supporting and positioning the casing, and forming a substantially impermeable barrier, or cement sheath, which isolates the wellbore into subterranean zones. Thus, the undesirable migration of fluids between zones is prevented after primary cementing.
Changes in pressure or temperature in the wellbore over the life of the well can result in compromised zonal isolation. Also, activities undertaken in the wellbore, such as pressure testing, well completion operations, hydraulic fracturing, and hydrocarbon production can affect zonal isolation. Compromised zonal isolation is often evident as cracking or plastic deformation in the cement composition, or de-bonding between the cement composition and either the wellbore or the casing.
Cement compositions are made chiefly of cement. Due to its incompressible nature, neat cement is undesirable for use where there is a chance of expansion or contraction in the wellbore. Cement has a high Young's modulus, and fractures at slight strains when subjected to stresses (“brittle failure”). When the imposed stresses exceed the stress at which the cement fails, the cement sheath can no longer provide zonal isolation. To lower the Young's modulus of cement compositions, sodium silicate is added, which results in a composition having a high water to cement ratio. However, such silicate treated cement compositions (“water-extended slurries”) suffer from low compressive and tensile strengths.
Therefore, a cement composition that can provide greater elasticity and compressibility, while retaining high compressive and tensile strengths, is desirable for primary cementing.